1. Field of the Invention
The present invention relates to a printed circuit board and a method of manufacturing the same.
2. Description of the Background Art
Flexible printed circuit boards (hereinafter referred to as FPC boards) are used in electronic equipment such as communication devices and computers. Differential transmission paths for transmitting differential signals at high speed are provided on an insulating layer, for example, in the FPC board. The differential transmission path is composed of a pair of transmission lines.
The pair of transmission lines needs to have equal electrical lengths in order to prevent the differential signals transmitted through the pair of transmission lines from being distorted. The pair of transmission lines cannot be linearly formed in parallel to each other in many cases because of limitation for circuit arrangement on the FPC board. Therefore, each transmission line is provided to be curved or bent on the FPC board. In such a case, the length of each transmission line needs to be adjusted such that the pair of transmission lines has the equal electrical lengths.
In a circuit board described in JP 2007-173371 A, for example, part of each conductor line is provided with a zigzag wiring portion that is indirect and extended in order to cause the electrical lengths of a plurality of conductor lines to be equal. In such conductor lines, the indirect and extended zigzag wiring portion and other portions having linear shapes have different characteristic impedances because of difference in capacitive coupling. This results in discontinuous characteristic impedances of the transmission lines, leading to reflection and distortion of the signals.
JP 2007-173371 A describes adjustment of the width of the indirect and extended wiring portion such that the roundabout and extended wiring portion and the other portions have equal characteristic impedances.
In recent years, flexibility in dimensions of transmission lines has been restricted as the transmission lines are more densely arranged in FPC boards. This makes it difficult to precisely adjust the width of the transmission line at an arbitrary portion in its longitudinal direction to a desired value. Accordingly, continuity of the differential impedance in the differential transmission path cannot be ensured by precisely adjusting the widths of the pair of transmission lines of the differential transmission path.